We propose to study the neural interaction that occurs between the antagonistic cardiovascular reflexes which arise from chemoreceptors and baroreceptors in dogs. Experimentally these reflexes will be simultaneously stimulated either by electrical stimulation of the carotid sinus and aortic nerves or by natural excitation of these receptors by changes in pressure and PO2-PCO2. During stimulation of these nerves with bursts of stimuli the effect of stimulation pattern, synchrony between the bursts and the burst duration on the integrated sympathetic outflow to the different vascular beds will be studied. The vascular response in the perfused hind limbs or in the perfused kidneys will be used as one method of monitoring sympathetic activity. Recording of neural activity in the sympathetic fibers will also be used to monitor changes in response to electrical stimulation. In addition to the studies in these vascular beds, studies relating to the general distribution of the cardiac output during hypoxic excitation of the chemoreceptors at different levels of carotid sinus pressure will be made. In these studies flow in the individual vascular beds will be made by using radioactive microspheres.